FIG. 1 shows timing sequence of a conventional monochrome composite video blanking sync signal 10, which mainly comprises two signal components, i.e., a luminance signal 12 and a horizontal synchronization signal 14. The luminance signal 12 comprises video saturation (luminance) information, and the horizontal synchronization signal 14 controls a display signal scan operation on a screen (or a television screen). Referring to FIG. 1, since a voltage level of the horizontal synchronization signal 14 is lower than that of the luminance 12, a conventional video signal decoder provides a slicing voltage level 16 crossing the horizontal synchronization signal 14, and compares voltage levels of the composite video blanking sync signal 10 with the slicing voltage level 16 to retrieve the horizontal synchronization signal 14 of the composite video blanking sync signal 10. However, since falling edges and rising edges of the horizontal synchronization signal 14 are distorted due to noise interferences occurring in transmission of the composite video blanking sync signal 10, the horizontal synchronization signal retrieved by the conventional video signal decoder is not synchronous with the horizontal synchronization signal 14 thereby creating distortion in a video on the screen. In other words, the distortion is created due to jitter between the horizontal synchronization signal retrieved by the video signal decoder and the horizontal synchronization signal 14, and the jitter is caused by the composite video blanking sync signal 10 disturbed by noise interferences. However, since noises are random signals, it is infeasible to eliminate the noises interferences in transmission of the composite video blanking sync signal 10. Accordingly, a solution for more effectively and accurately retrieving the horizontal synchronization signal 14 of the composite video blanking sync signal 10 to eliminate distortion is in need.